Plastic materials, such as PET (polyethylene terephthalate), have been used for some time now to make containers. This is particularly true for food-containing containers, especially drinks. Although said containers can be of different types, they shall be referred to hereinafter generically as bottles, which happen to be the most popular kind.
Two main production processes are used to make plastic bottles; both processes involve making an intermediary product, the so-called “preform”. The first type of process is a single-stage process and comprises injection and blow moulding machines forming a single-stage plant. The term “single-stage” is appropriate because the plant carries out the complete transformation process of the plastic in a continuous and automatic manner: from the pellet state to the finished, ready-to-use containers.
The second type of process is essentially the same as the first except that it is a “multistage” process: after the injection moulding stage, the obtained preforms are pre-cooled to a specific temperature and, then, stored in an intermediate storage place to be cooled further until they reach room temperature. The blow moulding stage of the process is carried out later and, obviously, requires the preforms to be heated beforehand so that the resin is soft enough to be shaped.
The first stage of the production process for making containers (i.e. injection moulding to produce preforms) utilizes moulds (specifically, split moulds—each made up of two mould halves—that open and close) with many injection mould cavities. The resin in the molten state is injected into said cavities and remains in the closed mould to cool for a specific amount of time until the preform has solidified enough to allow safe handling without deformation. Since the preform occupies the mould during cooling and solidification, it follows that the longer the preform is cooled inside the mould, the slower the moulding process. In fact, the mould can only be used in the next cycle after it has been opened and freed of the preforms.
For the sake of completeness, it should be mentioned that the complete injection moulding stage involves closing the moulds, injecting the resin in the molten state into the injection mould cavities, cooling the resin to solidify the structure of the preforms enough to enable handling, opening the moulds, and ejecting/removing the preforms. This stage, which directly affects and limits the productivity of the container production plant, is the so-called “bottleneck” of the production line and lasts longer than the other stages carried out by the plant. Furthermore, it is very difficult to shorten the duration of this stage; thus, improvements to the process have focused on shortening the cooling time inside the mould as much as possible and completing a significant part of the cooling after ejecting the preforms from the mould.
The increasing demand for plastic bottles, and for plastic containers in general, and the need to boost productivity to remain competitive mean that both single-stage and multistage moulding machines must minimize the amount of time the moulds remain occupied during the injection and cooling operations.
State-of-the-art injection and blow moulding plants try to accomplish this by ejecting the preforms as soon as possible after the injection step, completing cooling on special cooling plates fitted with special holders or cavities. Known types of cooling plates may comprise means for force cooling the preforms by circulating cooling fluids.
As was already mentioned, the cooling plate is fitted with a number of holders, having a shape that normally complements the contour of the preforms; the holders are held in position and cooled by a system that circulates a cooling liquid. The advantage of using said holders is that the deformation of the structure of the preforms is almost entirely eliminated during the cooling step.
Another means used to accelerate even further the injection moulding cycle in known injection and blow moulding plants is to equip the cooling plate with extra cooling holders (for example, double or triple the number of injection mould cavities). In this way, while an injection moulding cycle is being carried out in the mould, the preforms produced and ejected during the previous cycle(s) are being cooled in the holders of the cooling plate. Thus, by choosing the appropriate number of holders, the cooling stage can last approximately twice or three times the duration of the injection moulding cycle in the mould. Patent IT-PN2000A000006 describes an injection moulding plant of the type described above.
Unfortunately, said plant is somewhat complex to make and control. Furthermore, it requires a large number of components that make it expensive to construct and maintain.